A speaker system comprises, for example, a speaker and a cabinet that supports the speaker. A speaker system having superior acoustic characteristics can emit into free space sound waves having frequencies extending over a wide range, from the treble range to the bass range. Consequently, speaker systems currently in use possess a plurality of lowest resonant frequencies according to a plurality of speakers being provided therein, these speakers having a similar structure but of different sizes. Accordingly, speaker systems can emit sound waves having frequencies extending over a wide range.
However, since sound waves are emitted by vibrations of the diaphragm (for example, a cone-shaped diaphragm) of a speaker, phases of sound waves in front of and behind the diaphragm become mutually opposite. Consequently, sound waves emitted from the front of the diaphragm and those emitted from the rear of the diaphragm mutually cancel each other out. This mutual cancelling out resulting from mutually opposite phases is remarkable particularly in the bass range.
In order to enhance bass range sound waves, a bass enhancing member such as a passive radiator or a bass reflex duct are provided in the cabinet of the speaker system. Other bass enhancing devices may be added if the bass enhancing member does not satisfactorily enhance the bass range sound waves.
A conventional bass enhancing device enhances a specified audio-frequency range in a boost circuit of an amplifier, or carries out MFB (Motional Feed Back) to the amplifier according to an output of a vibration detecting sensor provided in the speaker system that is driven by the amplifier. Where such a bass enhancing device is provided, the acoustic characteristics of the speaker system improve from the treble range to the bass range.
However, in conventional bass enhancing devices, in the case where a resonant frequency of the diaphragm falls within the specified audio-frequency range enhanced by the boost circuit, the speaker unit including the diaphragm are driven by electrical power enhanced at a vicinity of the resonant frequency of the diaphragm. As a result, a mechanically and acoustically undesirable problem arises whereby the vibrating system is damaged due to excessive amplitude of vibration or, even if no damage occurs, sound tends to become distorted in the vicinity of the resonant frequency. This is because the vibrating system on being driven deviates from a state wherein linearity is maintained.
On the other hand, in the case where bass enhancing devices, wherein MFB is carried out to the amplifier according to the output of the vibration detecting sensor, use a speed-type MFB method, the damaging effect of excessive amplitude of vibration is reduced since a damping power exerted on the diaphragm increases. However, some sort of vibration detector must be attached to the vibrating system of the speaker unit that is driven by the amplifier. As a result, the weight of the vibrating system increases due to the vibration detector being attached thereon and problems appear such as reduced vibration efficiency of the speaker unit, as well as a more complicated structure of the speaker unit.